Electrical conductivity in granular media and Branly's coherer: A simple experiment

We show how a simple laboratory experiment can illustrate certain electrical transport properties of metallic granular media. At a low critical external voltage, a transition from an insulating to a conductive state is observed. This transition comes from an electro-thermal coupling in the vicinity of the microcontacts between grains where microwelding occurs. Our apparatus allows us to obtain an implicit determination of the microcontact temperature, which is analogous to the use of a resistive thermometer. The experiment also helps us explain an old problem, Branly's coherer effect, which was used as a radio wave detector for the first wireless radio transmission, and is based on the sensitivity of the conductivity of metal filings to an electromagnetic wave. (c) 2005 American Association of Physics Teachers.